compress

, uncompress

, zcat

- compress, uncompress files or display expanded files

Synopsis

compress [-fv/] [-bbits] [file]...

compress-c [-fv] [-bbits] [file]

uncompress [-fv] [-c | -/] [file]...

zcat [file]...

Description

compress

The compress utility attempts to reduce the size of the named files
by using adaptive Lempel-Ziv coding. Except when the output is to the
standard output, each file is replaced by one with the extension .Z,
while keeping the same ownership modes, change times and modification times, ACLs, and
extended attributes. The compress utility also attempt to set the owner and
group of file.z to the owner and group of file, but does
not fail if this cannot be done. If appending the .Z to the
file pathname would make the pathname exceed 1023 bytes, the command fails.
If no files are specified, the standard input is compressed to the
standard output.

The amount of compression obtained depends on the size of the input,
the number of bits per code, and the distribution of common substrings.
Typically, text such as source code or English is reduced by 50-60%.
Compression is generally much better than that achieved by Huffman coding (as used
in pack(1)) and it takes less time to compute. The bits parameter
specified during compression is encoded within the compressed file, along with a magic
number to ensure that neither decompression of random data nor recompression of
compressed data is subsequently allowed.

uncompress

The uncompress utility restores files to their original state after they have
been compressed using the compress utility. If no files are specified, the
standard input is uncompressed to the standard output.

This utility supports the uncompressing of any files produced by compress. For
files produced by compress on other systems, uncompress supports 9- to 16-bit
compression (see -b).

zcat

The zcat utility writes to standard output the uncompressed form of files
that have been compressed using compress. It is the equivalent of uncompress-c.
Input files are not affected.

Options

The following options are supported:

-bbits

Sets the upper limit (in bits) for common substring codes. bits must be between 9 and 16 (16 is the default). Lowering the number of bits result in larger, less compressed files.

-c

Writes to the standard output; no files are changed and no .Z files are created. The behavior of zcat is identical to that of `uncompress-c'.

-f

When compressing, forces compression of file, even if it does not actually reduce the size of the file, or if the corresponding file.Z file already exists.

If the -f option is not specified, and the process is not running in the background, prompts to verify whether an existing file should be overwritten. If the response is affirmative, the existing file is overwritten. When uncompressing, does not prompt for overwriting files. If the -f option is not specified, and the process is not running in the background, prompts to verify whether an existing file should be overwritten. If the standard input is not a terminal and -f is not specified, writes a diagnostic message to standard error and exits with a status greater than 0.

-v

Verbose. Writes to standard error messages concerning the percentage reduction or expansion of each file.

-/

When compressing or decompressing, copies any extended system attributes associated with the source file to the target file and copies any extended system attributes associated with extended attributes of the source file to the corresponding extended attributes associated with the target file. If any extended system attributes cannot be copied, the original file is retained, a diagnostic is written to stderr, and the final exit status is non-zero.

Operands

The following operand is supported:

file

A path name of a file to be compressed by compress, uncompressed by uncompress, or whose uncompressed form is written to standard out by zcat. If file is -, or if no file is specified, the standard input is used.

Usage

See largefile(5) for the description of the behavior of compress, uncompress, and
zcat when encountering files greater than or equal to 2 Gbyte ( 231
bytes).

Environment Variables

See environ(5) for descriptions of the following environment variables that affect the
execution of compress, uncompress, and zcat: LANG, LC_ALL, LC_COLLATE, LC_CTYPE, LC_MESSAGES, and NLSPATH.

Affirmative responses are processed using the extended regular expression defined for the
yesexpr keyword in the LC_MESSAGES category of the user's locale. The locale
specified in the LC_COLLATE category defines the behavior of ranges, equivalence classes, and
multi-character collating elements used in the expression defined for yesexpr. The locale
specified in LC_CTYPE determines the locale for interpretation of sequences of bytes
of text data a characters, the behavior of character classes used in the
expression defined for the yesexpr. See locale(5).

Exit Status

The following error values are returned:

0

Successful completion.

1

An error occurred.

2

One or more files were not compressed because they would have increased in size (and the -f option was not specified).